The present invention relates to a method for manufacturing a magnetic recording medium in which an extrusion-type coating apparatus is used to uniformly coat a flexible support (hereinafter referred to as a "web") made of a material such as plastic film, paper, or metal foil, with a magnetic coating solution to form at least the uppermost layer of the recording medium.
Numerous apparatus are known in the art to form a magnetic layer on a recording medium by known methods. For example, an extrusion-type coating apparatus, a curtain flow-type coating apparatus, a blade doctor-type coating apparatus, and a slide coat-type coating apparatus can all be used to coat a web with a coating solution to form a magnetic layer on a recording medium. Examples of the above apparatus are disclosed in Japanese Unexamined Patent Application Nos. 238179/1985, 88080/1988, 174965/1990 and 265672/1990, and Japanese Patent Application Publication No. 46186/1990. Of the known apparatus and methods, the extrusion-type coating apparatus is extensively employed in many industrial fields because it is superior in uniformly forming the layer on the web.
In the operation of the extrusion-type coating apparatus operates, while a web is run along the back edge surface and the doctor edge surface of the coating head of the apparatus, a coating solution is continuously discharged or extruded through a slot between the back edge surface and the doctor edge surface to coat the surface of the web.
Recent developments in recording media have presented difficult conditions for manufacturers to meet. For example, recently higher density recording media have been achieved by greatly increasing the number of layers formed on the web. In order to increase the number of layers on the web, the thickness of each layer must be reduced. In addition, new materials such as magnetic oxide particles or barium ferrite materials high in S.sub.BET value have been used to improve the magnetic characteristics of the media. These materials often require coating solutions with an increased viscosity. In addition, many manufacturers have been required to increase the speed of applying coating solutions to a web in order to increase the amount of production of the magnetic media per unit time.
The extrusion-type coating apparatus described above has many difficulties in meeting the above conditions. Indeed, the above-described method employed by the extrusion-type coating apparatus is able to achieve only a narrow range of satisfactory coating conditions, and therefore it is rather difficult to obtain satisfactory conditions for application of a coating solution. Moreover, these conditions are conventionally determined by trial and error. The determination of the coating conditions in this manner, however, lowers the yield of coating. As a result, the production is low in efficiency and the recording media produced is often unstable in quality.
Furthermore, because of the high aggregation of the coating solution, the layer formed on the web is lowered in surface characteristics and, therefore, it is difficult using the coating method to form a layer on a web which is stable in surface characteristics. For example, after a base layer of coating solution is formed on the web, a magnetic coating solution is applied to the base layer with a thickness of 4 .mu.m or less while the base layer is still wet. If the magnetic coating solution is abruptly stretched near the opening of the slot, vortices and stagnant solution are generated around the opening of the slot. As a result, the interface between the base layer and the magnetic layer becomes irregular and foreign substances trapped in the coating solution or caught by the web are apt to form stripes on the coating layer. Such defects in the coating are unacceptable because they adversely affect characteristics of the medium such as the electromagnetic conversion characteristics (e.g., signal output and signal-to-noise (S/N) ratio).